Internal combustion engines, including compression-ignition, spark-ignition engines, and other engines known in the art, may discharge a mixture of water vapor, carbon dioxide, and air pollutants during operation, the mixture commonly referred to as exhaust and the pollutants as emissions. The emissions may include gaseous and solid compounds, including carbon monoxide (CO), unburned hydrocarbons (“UHCs”), oxides of nitrogen (hereafter “NOx” to include nitric oxide (NO) and nitrogen dioxide (NO2)), particulate matter (“PM”) such as soot, and sulfur compounds among others. Regulatory emission standards limiting the release of such polluting emissions have become increasingly stringent, making the efficient and effective reduction of emissions a relatively important performance criterion for modern internal combustion engines. As a result, machines powered by internal combustion engines are typically equipped with exhaust aftertreatment systems, including such devices as catalysts, filters, adsorbents, and other devices, to remove regulated emissions from the exhaust and thereby comply with the applicable regulatory exhaust emission standards. Such aftertreatment systems may include one or more of a diesel oxidation catalyst (“DOC”), three-way catalyst, lean NOx catalyst, selective catalytic reduction (“SCR”) catalyst, a filtration component, either catalyzed or uncatalyzed (e.g., a diesel particulate filter (“DPF”)), and a cleanup catalyst (e.g., an ammonia oxidation catalyst).
As the amount and types of emissions from an engine may vary depending on the type, size, and/or operating conditions of the engine, so the effectiveness of exhaust aftertreatment devices can vary with engine operating conditions, particularly the temperature of the exhaust generated by the engine. Generally, each type of aftertreatment device has a minimum temperature, commonly referred to as “activation temperature,” that enables effective and efficient operation of the aftertreatment device. The rate of emissions removal is generally not sufficient to meet regulatory requirements when the temperature of an aftertreatment device is less than the activation temperature. Moreover, an aftertreatment device may become fouled by emissions when operated below the activation temperature. For example, FIG. 7 shows the effect of temperature on an exemplary SCR device having an activation temperature of around 240° C., illustrating behavior typical for conventional aftertreatment devices. As shown in FIG. 7, below the activation temperature a SCR device tends to adsorb UHCs, which form deposits that clog or foul the SCR device in a way that is difficult to reverse. Above the activation temperature, the SCR device tends to desorb UHCs, which may evaporate off the surfaces of the SCR device at elevated temperatures.
During operation of the engine, the exhaust temperature varies depending on such factors as the engine speed and load and, to some extent, the ambient temperature. Generally, exhaust temperature decreases quickly when the engine is operated under light load, low speed conditions, such as idle. Other operating conditions, such as operating at low speeds in winter weather, can further result in low exhaust temperatures.